Control Flow & Loops

Control flow statements allow your program to make decisions and execute blocks of code repeatedly, based on specific conditions.

1. Conditional Statements

Conditional statements determine which code block is executed based on the evaluation of a Boolean expression (True or False).

A. The if Statement

The if statement is used to execute a block of code only if a condition is met.

var
  Age: Integer;
begin
  Age := 25;
  
  if Age >= 18 then
  begin // If block contains more than one statement, 'begin/end' is required
    ShowMessage('You are an adult.');
  end
  else
    ShowMessage('You are a minor.');
end;

Single Statement Rule: If the then or else clause contains only a single statement, the begin and end keywords are optional. Using begin/end is often recommended for readability.

B. The if...then...else Statement

Used to define two distinct paths of execution: one if the condition is True, and another if it is False.

var
  IsMember: Boolean;
begin
  IsMember := True;
  
  if IsMember then
    TotalCost := 50.00 // No semicolon before 'else'
  else
    TotalCost := 75.00;
end;

C. Nested if and else if

You can chain multiple conditions using nested if statements or the cleaner else if structure.

var
  Score: Integer;
begin
  Score := 85;
  
  if Score >= 90 then
    Grade := 'A'
  else if Score >= 80 then // Note: Object Pascal uses 'else if' or 'else begin if'
    Grade := 'B'
  else
    Grade := 'C';
end;

D. The case Statement

The case statement provides a cleaner alternative to multiple if...else if structures when you are testing a single variable against multiple, specific constant values.

var
  Day: Integer;
begin
  Day := 3;
  
  case Day of
    1: DayName := 'Monday';
    2, 3, 4: DayName := 'Weekday'; // Handles multiple values
    5: DayName := 'Friday';
    6..7: DayName := 'Weekend'; // Handles a range of values
    else // Optional block for values not matched
      DayName := 'Invalid Day';
  end; // The 'end' closes the 'case' block
end;

2. Iteration (Loops)

Loops are used to execute a block of code repeatedly until a specific condition is met or a set number of times.

A. The for Loop (Fixed Iteration)

The for loop is used when you know exactly how many times you need to execute the loop.

• Counting Up (Ascending): Uses the keyword to.
• Counting Down (Descending): Uses the keyword downto.
• Loop Variable: The loop control variable (e.g., i below) must be declared, usually locally in the procedure’s var block.

procedure TForm1.CountUp;
var
  i: Integer;
  Result: Integer;
begin
  Result := 0;
  
  // Counts i from 1 to 10
  for i := 1 to 10 do
  begin
    Result := Result + i;
  end; // end of the loop block
  
  ShowMessage('Total: ' + IntToStr(Result));
end;

B. The while Loop (Pre-test)

The while loop checks the condition before executing the loop body. If the condition is initially False, the loop body never runs.

var
  i: Integer;
begin
  i := 0;
  
  while i < 5 do
  begin
    i := i + 1; // Must include logic to eventually make the condition False
    Memo1.Lines.Add(IntToStr(i));
  end;
end;

C. The repeat...until Loop (Post-test)

The repeat...until loop executes the loop body at least once before checking the condition. The loop continues until the condition becomes True.

var
  i: Integer;
begin
  i := 10;
  
  repeat
    ShowMessage('Current count: ' + IntToStr(i));
    i := i + 1;
  until i > 10; // Loop executes once (since i=10 is not > 10), then terminates.
end;